WASHINGTON: China-linked hackers appear to have gained access to sensitive background information submitted by US intelligence and military personnel for security clearances that could potentially expose them to blackmail.

In a report citing several US officials, the news agency said data on nearly all of the millions of US security-clearance holders, including the Central Intelligence Agency, National Security Agency and military special operations personnel, were potentially exposed in the attack on the Office of Personnel Management.

It said more than 2.9 million people had been investigated for a security clearance as of October 2014.

The OPM did not immediately respond to requests for comment, but a senior US official confirmed that US investigators had discovered a separate attack on the OPM that targeted sensitive information about government employees similar to a hacking incident revealed last week.

The official, who spoke on condition of anonymity, could not confirm that the information obtained was from US intelligence and military personnel but did say it was "a different set of OPM systems and data" to that of the hack disclosed last week and did involve background data and security clearances.

A source familiar with the investigation said US investigators suspected a similar Chinese link to the other hacking incident.